Cargo planes played a crucial role in getting supplies to the front lines in Vietnam.

American pilots had to land heavy, laden down aircraft in remote areas far from existing

air strips. They couldn't touch down on rough, dusty terrain, and constructing permanent

runways out of concrete was impractical and slow. The answer was simple. Big sheets of

corrugated aluminium that could be driven to the makeshift air strips.

Weighing about the same as a small adult, they were light enough that a few burly soldiers

could lay them down without heavy machinery, but sturdy enough for large planes to land

on. The mats were woven together to make runways nearly two miles long.

Decades later, surplus mats were put to use nearer to home. They stood upright to form

a barrier along America's southern border. Many are still there today. Any piece of corrugated

sheet metal on the Mexican border was probably once destined for the battlefields of Vietnam.

The remnants of an old defeat marking the site of a current crisis that's becoming harder

to solve. This is Chex and Balance.

I'm John Prado, the economist's US editor and each week we take one big theme shaping

American politics and explore it in depth.

Today

What's happening at the US Mexico border?

Vaccinated travelers can now visit America for the first time since the pandemic began.

At the southern border, legal crossings have only just restarted. But this year, border

patrol agents have arrested more people crossing illegally than at any time in the past

20 years. President Biden promised to undo his predecessors' immigration policies, but

the situation on the ground is out of control.

...